1. Field of the Invention
The invention is based on a device for determining at least one parameter of a medium flowing in a line, and particularly for measuring a parameter of a flow into an internal combustion engine.
2. Description of the Prior Art
DE 197 35 891 A1 has disclosed a measuring body, which can be inserted into a clean conduit of an intake line of an internal combustion engine and is used to determine the mass of intake air; the measuring body has a flow conduit and a measurement conduit, which is essentially inclined in relation to a longitudinal axis of a line and is comprised of an S-shaped deflection conduit adjoining said line. A measuring element is disposed in the measurement conduit. As has been disclosed, for example, by DE 43 38 891 A1 and U.S. Pat. No. 5,452,610, the measuring element can be designed as a micromechanical sensor part with a dielectric membrane. The measuring element can become contaminated by solid particles that penetrate through the air filter and by water permeation through the air filter into the intake line, e.g. as a result of roads that are wet with rain. Natural components of dissolved salts contained in this sprayed water then cause a drift in the characteristic curve due to the buildup of salt encrustation on the membrane of the sensor part.
The inclination of the measuring body does in fact produce a protected leeward region, but a part of these solid particles or fluid particles still get into the measurement conduit.
DE 197 35 664 A1 has already disclosed a device in which the measuring element is disposed inside a tubular body that the medium flows through, where an upstream end of the tubular body extends into a filter chamber and has inlet openings disposed there on a circumferential surface in order to reduce the action of dirt particles or water droplets on the measuring element. Particularly with very dirty air and a high water content in the intake air of the internal combustion engine, there is the danger that the air filter will become saturated with water, which then seeps through the filter mat and entrains dirt particles with it. On the downstream side of the air filter, the actually clean side, there is now the danger that the intake air will once again entrain dirt particles and water droplets from the filter surface, which will then be undesirably deposited on the measuring element and will lead to erroneous measurements or to a failure of the measuring element. The tubular body according to the prior art does reduce the danger of deposits by placing inlet openings on the circumferential surface, but a correspondingly long design of the tubular body produces an undesirable pressure drop, which results in a reduction in measuring sensitivity. In addition, the reduction of an action of fluid/solid particles on the measuring element can hardly be assured with a very high fluid influx of approx. 20 liters/hour.
DE 196 52 753 A1 has disclosed a device with a measuring element, which contains a flow rectifier and a screen in order to stabilize a measurement signal. However, no additional screen or element is used to protect the measuring element from fluids or solid particles.
The use of a deflecting screen in a line has also been proposed for separating fluid particles from a flowing air or gas. A deflecting screen of this kind, which is connected upstream of an inner tube or in the line, influences the air/water mixture flowing toward the measuring element in such a way that the fluid particles are conveyed against a tube wall or a line wall while the air remains in a center of the inner tube.
A different behavior is produced when a mixture of air and dust passes through a deflecting screen in the line. The dust does not form a wall film like a fluid, but is reflected against the line wall, where the principle of the angle of incidence being equal to the angle of reflection applies. Since the measuring element is disposed a certain distance from the deflecting screen, a certain portion of the reflected solid particles collide with the measuring element disposed downstream of the deflecting screen.